Social anxiety disorder (SAD) is a common and debilitating disorder, associated with tremendous personal and societal costs. Although efficacious treatments exist for SAD, relatively little research has been devoted to identifying specific mechanisms involved in the etiology of SAD. Empirical research suggests that biased attention toward social threat-relevant information in the environment is an important causal factor in the maintenance of SAD; however, relatively little is known about the role of attention bias to threat in the development of SAD. Given research suggesting that attention bias to threat is a reactive process that may be overridden by more strategic level cognitive processes, such as attentional control, the aims of this study are to examine the moderating roles of risk for SAD and attentional control on the relationships between attention bias to threat and (1) psychological and biological social stress reactivity and (2) development of SAD. For many, the onset of SAD occurs during late adolescence/early adulthood, during which one major stressor is entrance into college. Given that social anxiety increases from the summer prior to matriculation to during the first semester of college, participants in the proposed study will be college freshmen either at low- or high-risk for developing SAD but who do not meet diagnostic criteria for SAD. During the summer prior to matriculation, participants' attentional control and attention bias to social threat will be assessed. Participants will complete second and third assessments during the fall and spring semesters, during which their biological (i.e., neuroendocrine) and psychological social stress reactivity will be measured during a behavioral task. Changes in social anxiety severity across the course of the first year in college will be assessed with self-report questionnaires and a second diagnostic interview for SAD conducted at the third assessment. The prospective, high-risk design of the proposed study has the potential to illuminate cognitive mechanisms involved in social stress reactivity and the development of SAD. Given the high prevalence and burden of SAD, research is needed to elucidate causal factors at play in the development of SAD. Results from this study may help to improve early identification of at-risk individuals as well as inform innovative prevention programs, such as programs that re-train biased attention or improve effortful control of attention.